Provide an entity lookup plugin with support for default lookup value.

Some thoughts. First great work here. Generally useful plugin. I wonder if we could make it even more useful? Also, we should add some tests. See other examples in this module. Since you are interacting with entities, it will probably need to be a kernel test.

Setting to NW for tests and the suggestion to make this more generally useful.

1. +++ b/src/Plugin/migrate/process/DefaultUser.php
   @@ -0,0 +1,50 @@
   +class DefaultUser extends ProcessPluginBase {
   

   Can we extend DefaultValue and pass things off to it after loading the uid?

   Actually, what if we made this a 'default_value_plus' plugin? And provided an entity type, lookup_value, lookup_key, etc so we can then use an entity query to look for a default value based on any query-able condition.

   Example:
   entity_type: user
   lookup_key: username
   lookup_value: "john smith"
   default_value_key: uid

   The above names are just examples. If you have better suggestions on naming, feel free to use them instead.

1. +++ b/src/Plugin/migrate/process/DefaultValuePlus.php
   @@ -0,0 +1,46 @@
   + *   id = "default_value_plus",
   ...
   +class DefaultValuePlus extends EntityLookup {
   

   Naming is hard. Would default_entity_value / DefaultEntityValue make sense more to convey a more direct meaning?

Thanks for your contributions!